The present invention relates to an electrophotographic photosensitive member and an electrophotographic apparatus which employs the same.
An electrophotographic photosensitive member is basically formed from a supporting member and a photoconductive layer formed on the supporting member. However, an intermediate layer provided between the photoconductive layer and the supporting member is effective for covering the defects of the supporting member, improving the coating of the photoconductive layer, improving adhesion between the photoconductive layer and the supporting member, protecting the photoconductive layer from being electrically isolated, improving the charging of the photoconductive layer and improving charge injection from the supporting layer to the photoconductive layer. Hence, it is required that the intermediate layer of the electrophotographic photosensitive member is coated and adhered well, exhibits a high mechanical strength, is appropriately conductive and offers an electrical barrier.
The types of intermediate layers which have been proposed are as follows:
(i) A thin resin film containing no conductive filler, PA1 (ii) A thin resin film containing a conductive filler, and PA1 (iii) A laminated layer in which the thin film of (i) is laid on top of the thin film of (ii).
Since the thin film of (i) contains no conductive filler, it has a high electric resistance. Further, in order to cover the defects of the supporting member, the film must be made thick. The thin film of (i), then, has a disadvantage in that the residual potential at the initial stage of use thereof or when the thin film is used repeatedly, is high. To make the thin film of (i) practical, the defects of the supporting member must be reduced and the thickness of the intermediate layer must be reduced greatly.
The thin film of (ii) or (iii) can be made appropriately conductive because of the dispersed conductive filler. However, if the dispersion of the conductive filler is degraded, then the electrical characteristics of the intermediate layer, such as an electric resistance or a dielectric constant thereof, changes. Such variations adversely affect the potential characteristics or image formation. A degraded dispersion also degrades the uniform flatness of the film surface, causing coating defects to occur. Further, the degraded dispersion of the filler reduces the adhesion of the intermediate layer to the supporting member and the mechanical strength of the intermediate layer.
Examples of the conventional conductive fillers of the intermediate layer include a conductive metal filler (disclosed in Japanese Patent Laid-Open No. sho 58-181054), a conductive metal oxide filler (disclosed in Japanese Patent Laid-Open No. sho 54-151843), and a conductive metal nitride filler (Japanese Patent Laid-Open No. hei 1-118848).
However, an electrophotographic photosensitive member which employs, as the filler of the intermediate layer, any of the conventional conductive fillers is highly dependent on changes in the ambient environment, such as temperature or humidity. Accordingly, employing conventional conductive fillers makes it difficult to manufacture a photosensitive member which exhibits excellent potential and image formation properties stably under various environments, including a low-temperature and low-humidity environment and a high-temperature and high-humidity environment. When such a photosensitive member is repeatedly used under, for example, a low-temperature and low-humidity environment where the electrical resistance of the intermediate layer increases, then electric charge accumulates in the intermediate layer, thus increasing the residual potential and the potential of a bright portion. Under a high-temperature and high-humidity environment where the electrical resistance of the intermediate layer is reduced, the electrical barrier property of the intermediate layer is reduced. This accelerates carrier injection from the supporting member and thus reduces the potential of the dark portion when the photosensitive member is used repeatedly. Consequently, the density of a formed image is reduced under a high-temperature and high-humidity environment, and unnecessary black spots or fog readily occurs in the developed image when employing a reversal development type of electrophotographic printer.
The present inventors consider that the characteristics of the photosensitive member change during variations in the environment when utilizing a degraded dispersion of the conductive filler. In other words, a degraded dispersion of the conductive filler in the intermediate layer creates local changes in the electrical resistance thereof, which, in turn, make the potential or image characteristics of the photosensitive member very sensitive to changes in the environment.